Modeling Investigation of Water Partitioning at a Semi-arid Hillslope

1. Modeling Investigation of Water Partitioning at a Semi-arid Hillslope Huade Guan, John L. Wilson Dept. of Earth and Environmental Science, NMT Brent D. Newman Earth and Environmental Sciences Division , LANL Jirka Simunek Department of Environmental Sciences, UCR AGU Fall, 2003

2. Acknowledgements • The analysis in this presentation was supported by SAHRA– the NSF Science and Technology Center for Sustainability of semi-Arid Hydrology and Riparian Areas • Site data was collected as part of the Los Alamos Environmental Restoration Project • Modifications to the numerical code were funded by NSF grant, SAHRA, and Swedish Research Council

3. To allow distributed mountain block recharge occur, you need water to enter the bedrock at the hillslope scale. Motivation: Mountain Front Recharge Is distributed mountain block recharge significant?

4. Two primary controls on percolation Precipitation • Water availability @ the soil-bedrock interface • Bedrock permeability • Percolation  Min (water availability, Ksat_rock) Soil water Soil Bedrock Hillslope scale Preliminary (generic) simulations

5. Highly permeable volcanic bedrock. Apparently little percolation reaches the bedrock (Wilcox et al., 1997). Water availability controls percolation, not bedrock permeability. Why? Macropore flow appears to occur in the low permeability soil horizon (Newman et al., 2003). Field site:ponderosa pine hillslope at a semi-arid area Figures from Wilcox et al. (1997)

6. Objectives of this study • Use numerical modeling to synthesize the observations and previous generic simulations • Is the percolation into the bedrock really negligible? • It wasn't directly observed, just inferred. • If it is negligible, why? • What impedes downward movement of water into the highly permeable tuff? • For what situations will percolation to bedrock • becomesignificant for this climate? • …and with this permeable volcanic bedrock?

7. What we know and don’t know We know • Soil horizons and hydraulic parameters • Root density profile • Precipitation and other meteoric parameters • Soil moisture • Surface runoff and interflow • Root-derived macropore flow We don’t know • ET • Percolation

8. Modeling challenges • Modeling ET • System-dependent ET model • Appropriate root-water-uptake model • Modeling macropores • Root-derived macropores • Sub-parallel to the slope • Numerical issues • Highly non-linear, coupled processes • Dual permeability We used a modified version of HYDRUS-2D

9. P, PE, PT A Bw Bt CB R Root zone Seepage face 50cm Free drainage Hillslope setting Moisture profiles at three seasons, 1993 Figure from Wilcox et al. (1997)

10. PT PE h2 h3 T/PT E (wilting point) hmin h4 h1 Water potential Water potential ET modeling • ET accounts for 95% of the annual water budget (Brandes and Wilcox, 2000) • ET modeling

11. PT h2 h3 T/PT (wp) h4 h1 Water potential Calibration of ET modelillustrated using measured moisture profiles for 4 of 19 sampled days PE=50%, PT=50%, h4=-50m Root density A+Bw=0.59, Bt=0.4 PE=70%, PT=30%, h4=-50m Root density A+Bw=2.0, Bt=0.3 PE=70%, PT=30%, h4=-50m Root density A+Bw=0.65, Bt=0.35 PE=70%, PT=30%, h4=-15m Root density A+Bw=0.65, Bt=0.35 PE=70%, PT=30%, h4=-15m Root density A+Bw=2.0, Bt=0.3

12. Root D x b β z x θ Representing root-derived macropores 1. Annular root macropore aperture 3. Equivalent root dip angle 2. Radial root distribution

13. Conceptual models for macropore flow • Control: Model without macropores • Single continuum (sc) • Models with macropores • Single continuum with anisotropic K with three root dips (x1:1°, x2:15°, x3:30°) • Composite continuum (cc) • Dual permeability model (dp)

14. Simulated1994 water balance

15. Simulated and observed runoff No macropore (sc) Composite continuum (cc) simulated Observation observed Macropore, β=1° (x1) Macropore, β=15° (x2) Macropore, β=30°(x3)

16. Results of best-fit simulation(x2) Infiltration (cm) ET 48.5 46.0 Runoff Interflow Percolation 3.0 0 0.38 (0.7%P)

17. What happens if root-zone directly contacts the tuff? simulated observed Infiltration (cm) ET 48.5 (x2: 48.5) 39.3 (46.0) Runoff Interflow Percolation 3.0 (3.0) 0 (0) 5.0, 10.0%P (0.38, 0.7%P)

18. Conclusions • The simulated percolation across the soil-bedrock interface at this site is less than 1% of annual precipitation, in good agreement with previously inferred. • The simulation results are consistence with Wilcox et al’s (1997) alternative hypothesis that the CB horizon, without roots, behaves as a barrier to downward movement of water into the bedrock. • The results also indicates that sub-horizontal root-derived macropore flow increases the infiltration capacity and decreases surface runoff at this site. • In this climate, at a location with a shallower soil layer where the root zone contacts the highly permeable tuff, percolation can be as large as 10% of the annual precipitation.

19. The End

20. loam Sandy loam Implication about the ET model Feddes model overestimate ET loss based on the observed wilting point (h4). S-shape model is better if the numerical instability can be avoided.